Abstract
Eighteen new ts mutants of poliovirus have been isolated after a variety of mutagenic treatments, and their loci identified in relation to the previous genetic map. The map was only extended by 25%, and the physiological characters of the new isolates corresponded in all aspects tested with those of the previous isolates. Apparently single mutants at the extreme left of the map were defective in synthesis of both double- and single-stranded RNA, functions that do not co-vary in other mutants. Two procedures respectively predicted to induce mutations preferentially in the 5' and 3' regions of the genome gave isolates which all indicated that the structural protein region was nearest the 5' end. The loci fro resistance to dextran sulphate and to ethyl-2-methylthio-4-methyl-5-pyrimidine carboxylate both lie in the structural protein region.